Condoleezza Rice, the US secretary of state, is to fly to London next week to tackle an escalating row over Nato troop reinforcements for Afghanistan, amid worries that the entire international stabilisation strategy is in danger of failing.

Rice will also be talking to Gordon Brown and David Miliband, the foreign secretary, about finding a new candidate for the proposed UN "super-envoy" to coordinate aid efforts after President Hamid Karzai vetoed Lord Ashdown, who had been expecting to get the job.

Canada is already threatening to pull its troops out unless other countries do more. The US state department spokesman, Sean McCormack, told reporters in Washington yesterday. "I won't make a secret of the fact that we are encouraging all Nato allies to do everything they can in terms of contributing resources."

Rice will also discuss Iran and Iraq.

Nato defence ministers are to meet in the Lithuanian capital Vilnius next week to try to find 7,500 more troops to reinforce the 42,000 already in Afghanistan.

Alliance divisions burst into the open earlier yesterday with a US demand that Germany, whose forces are in the relatively stable north, send combat troops and helicopters to the volatile south.

Robert Gates, the US defence secretary, sent an "unusually stern" request to Berlin, the Süddeutsche Zeitung reported.

But Germany's defence secretary, Franz Josef Jung, refused to comply. "I keep to the view that we should continue and fulfil our mandate in Afghanistan," Jung said. "I believe our focus should continue to be in the north."

German chancellor Angela Merkel made clear that the limited mandate was "not up for discussion".

On Thursday Gates met similar opposition from his French counterpart, Hervé Morin, in talks in Washington. The mood in Paris and Berlin threatens a damaging replay of the transatlantic spats in the run up to the Iraq war five years ago.

But the immediate crisis has been triggered by Canada, which has threatened to bring home its 2,500 troops from Kandahar, next to Helmand province where British forces are fighting the resurgent Taliban insurgency, unless other allies send reinforcements.

Stephen Harper, the Canadian prime minister, told Gordon Brown this week that the "clear choice" laid out by an internal Canadian panel was that Canada would remain in Afghanistan beyond February 2009 only if allies supplied more combat troops for Kandahar and Canada acquired new equipment.

Gates angered Nato members last month when he complained that forces were still stuck in cold war mode and ill-prepared for counter-insurgency operations.

Wrangling over the number of boots on the ground coincides with a flurry of warnings that the entire effort to stabilise Afghanistan could fail because of resurgent Taliban violence and a looming humanitarian crisis. On Wednesday the former US Nato commander, General James Jones, suggested Afghanistan was in danger of becoming a "failed state" because there were "too few military forces and insufficient economic aid". Oxfam separately urged troop and aid-contributing countries to undertake "a major change in direction".

Lord Malloch-Brown, the Foreign Office minister, admitted yesterday that the situation was "very difficult". The UK, US, Canada and the Netherlands were all "re-tooling strategy to align it more realistically with the possibilities", he told BBC Radio 4's Today programme. "Which means more emphasis on not just a military victory but a political approach too in support of Karzai to win over those who've become disaffected in recent years, to win them back."

There was fury in London when Karzai criticised the performance of British troops in Helmand and blocked Lord Ashdown's appointment.

Conservative MPs are starting to question the wisdom of continuing to support the president. Ministers are braced for another critical report from the Commons international development select committee amid concern that popular support for the war will start to wane, especially as Karzai adopts an increasingly independent view.

Eighty-seven UK service personnel have been killed in Afghanistan since 2001, most of them in the past two years.